The largest volcanic eruption to have occurred in New Zealand since European settlement is the 1886 Tarawera Rift eruption. This eruption destroyed the famed Pink and White Terraces, coated the land for kilometres around with mud and volcanic ash and took the lives of 108 people in seven villages near the mountain. All the vegetation in the area, especially about Waimangu and Lake Rotomahana was devastated by the eruption. The vegetation seen today has invaded the area since the 1886 eruption.

Present day hydrothermal activity at Waimangu is associated with the south-western end of the 17km-long fracture formed on 10 June 1886 by the Tarawera Rift eruption. Surface fractures extend from the NE of Wahanga Dome on Mt Tarawera to Southern Crater at Waimangu (Fig.1). Tarawera volcano provides the heat source for the Waimangu hydrothermal activity.

Early morning on 10 June 1886 residents of Rotorua and surrounding districts were awakened by earthquakes which increased in frequency and intensity until shortly after 2am when the volcanic eruption commenced at Ruawahia Dome on Mt Tarawera. The eruption extended both NE and SW as the fissures opened. By 3.30am the eruption extended 17 km SW through the lake basins of Rotomakariri and Rotomahana to the region now known as Waimangu. Violent steam and lava explosions deepened and enlarged the Rotomakariri and Rotomahana basins to form Rotomahana Crater, destroying the famous sinter aprons of Te Tarata (White Terrace) and Otukapuarangi (Pink Terrace) along with other hydrothermal features at old Rotomahana. About 15 craters were created in the Waimangu area.

Today at Waimangu, one see’s hot springs and crater lakes that have developed in and about the 1886 craters. No surface hydrothermal activity had been previously recorded. Within a few months of the June eruption newly developed activity reportedly waned, but between 1888 and 1896 surface activity gradually increased and became permanently established.

Waimangu is worldly unique as it is the only volcano-hydrothermal system in the world whose origin can be pinned down to an exact day.

POST 1900 HYDROTHERMAL ACTIVITY

Late in 1900 Waimangu Geyser commenced erupting in the north-eastern portion of Echo Crater and continued semi-regularly to 1904 (Fig 2; Photo 1). Eruption heights up to 450 m are reported. To see Waimangu in eruption was the aim of early visitors to the Rotorua district. By the summer of 1902-3 a new tourist trip known as the ‘Round Trip’ was organised and continues today. In August 1903, resident guide Alf Warwrick launched a rowboat on the geyser’s lake as a result of a dare. He and his companion measured the lake depth at 48 feet (14.6m). The geyser’s activity became weaker after October 1904 and ceased erupting by November.

Following the demise of Waimangu Geyser there were no significant events until 21 February 1906 when a moderately large event formed the Mud Rift in Raupo Pond Crater (Fig 2). By 1912-13 small hydrothermal (steam) eruptions started to occur, particularly within Echo Crater; probably as a result of redirection of the subterranean heat to this area. On 12 April 1915 a further event occurred south of Waimangu Geyser basin (Fig 2). A large eruption on 1 April 1917, re-excavated and enlarged southern Echo Crater. An accommodation house 600m to the SW was destroyed and two lives were lost (Photos 2,3 & 5). By 26 June 1918 a lake, called Frying Pan, almost filled the craters formed by this eruption.

A further eruption occurred within Echo Crater on 29 August 1924, while in June 1951 an eruption occurred on the shore of Lake Rotomahana. Then on 22 February 1973 the Trinity Terrace area on the SE shore of Frying Pan Lake was destroyed. The most recent eruption was in May 1981 when a small event in Raupo Pond Crater created 2 new craterlets and destroyed the Mud Rift (Fig 2).

Results of scientific monitoring at Waimangu since 1971 have shown changes can occur both before and after these events. Sometimes changes like those which accompany eruptions are observed but no eruption follows. This makes forecasting very difficult, if not impossible.

WORLD-WIDE SIGNIFICANCE

Today, Waimangu is an area of diverse and intense hydrothermal activity containing two of the largest and most spectacular hot springs in the world. These are Frying Pan Lake, which occupies the 1917 sub-craters within Echo Crater and Inferno Crater Lake which occupies an 1886 crater blasted through the side of Mt Haszard (Photo 4). Waimangu also hosts a wide variety of unique thermal plants in many habitats, ranging from acid to alkaline soils, hot swamps and warm bare ground. All of New Zealand’s thermal plants are represented at Waimangu.

Many scientists (geologists, volcanologists, botanists) visit Waimangu and Rotomahana, attracted by the fact that it is the only thermal area in the world that has wholly formed in historic times. Scientists maintain extensive networks of monitoring equipment throughout the Valley, on Mount Tarawera, and the whole of the Ruapehu to White Island volcanic region. A feature of this region is the relative thinness of the Earth’s crust, about 10 km, compared with an average 20-25 km thickness in non-volcanic regions.

CYCLIC ACTIVITY AT WAIMANGU

Scientific instrumentation was installed at Waimangu in 1970 to investigate the hydrology of Frying Pan and Inferno Crater lakes. It has recorded the remarkable and unusual cyclic nature of these large hot lakes.

Figure 3: Plot of Frying Pan Lake overflow and Inferno Crater level showing the inverse relationship. Note how the overflow from Frying Pan increases as the water level in Inferno Crater falls.

Perhaps this cyclic hydrology was initially displayed by the Waimangu Geyser, which erupted semi-regularly from 1900-1904. Today Inferno Crater Lake and Frying Pan Lake show interrelated cyclic variations about a 38 day cycle (Fig 3). There is no other comparable cyclic activity known in hydrothermal systems in the world.

The lakelet occupying Inferno Crater has exhibited water level variations since at least 1901, when the lake level rose and fell in step with the Waimangu Geyser cycle. Today water level fluctuates from overflow to at least 8 metres below overflow. While this is occurring the volume of the overflow from Frying Plan Lake varies. This variation is the inverse of the lake level in Inferno Crater (Fig 3). That is, as Inferno Crater Lake water levels falls the overflow of Frying Plan lake increases, and when the water level in Inferno Crater rises the overflow from Frying Pan decreases.

The characteristics of the hydrology of the two lakes and the cyclic behaviour from 1971-1990 has been examined in detail by Government and University scientists. Studies of Frying Pan Lake show the overflow has decreased from 122 to 104 litre per second between 1972 and 1990; this correlates well with long term rainfall trends. The variation in the overflow (which is inversely related to the 38 day cycle of the water level of Inferno Crater Lake) is about 20 litre per second. Annually the overflow temperature ranges from 44 to 56oC (112 – 132oF) with a mean of 50oC (121oF). The local air temperature strongly influences the temperature of the discharge, due to heat loss from the lake surface.

The water level of Inferno Crater Lake has ranged from an overflow of 192 litre per second to 12.8 m below its overflow level and the temperature from 35 to 84oC (95-184oF). A cycle of 38 days is well established and four distinct stages are recognised within the cycle (Figs 3&4).

Figure 4: Plot of water level in Inferno Crater Lake showing the four stages of the cycle.

The scientific model for how Inferno Crater operates is very complex, but is similar to the model of a geyser. The driving force (motor) is pure steam at about 140m depth, with a cavity above it large enough to hold all the lake water (45 800m3). The steam heats the water which expands from its underground storage into the lake basin (Stage 1). This continues until the heat loss from the surface exceeds the heat input and the water level drops slightly. This process is repeated several times during Stage 2, slowly pushing the water level in the lake closer to overflow level. The water in the lake is accumulating energy from the steam at depth. When overflow level is reached water is poured from the lake down the overflow channel taking a large volume of water and stored energy in the lake with it. This causes an energy misbalance and the steam system fails, allowing the lake water to drain back into its underground storage cavity. The energy and water lost during the overflow stage is replaced from depth. Thus the cycle restarts.